1. Field of the Invention
This invention relates to a system for measuring the duration of a telephone call and more particularly to an economical, accurate electronic telephone call timing system capable of providing readily and independently variable call initial time and overtime rates.
2. Description of the Prior Art
Commonly, at the present time, timing systems utilized in an electromechanical telephone exchange central office for timing telephone calls are of the electromechanical type. Each timing system includes a synchronous motor which drives an elongated shaft extending adjacent a plurality of timing devices. Upon one of the timing devices being seized in response to the completion of a telephone call, a solenoid is energized to move a small drive gear longitudinally on the shaft into meshing engagement with a larger gear of the timing device. Gearing between the synchronous motor and the drive shaft, and the relative sizes of the drive gear and the larger gear driven thereby, are chosen so that the larger gear makes a single revolution in a time period corresponding to a maximum initial time period for a telephone call. During the rotation of the larger gear, a system of control cams thereon operates associated contacts to perform certain auxiliary control functions in the telephone exchange. In the event that the maximum initial time period is exceeded, the driven gear and its associated cams continue to revolve until the call is terminated, whereupon the gear is returned circumferentially to an initial start position by a coil-type biasing spring.
In these electromechanical timing systems, the initial time and overtime intervals for a telephone call are always identical because the same gear-cam assembly is used to measure both the initial time period and the overtime period. Further, to change the time interval involves changing the drive gearing for the control cams of each individual timing system, which requires that the timing system be taken out of service in order to make the desired changes.
U.S. Pat. No. 3,753,378, issued Apr. 6, 1971, to A. Leyburn et al, discloses a telephone call timing circuit in which pulses from a single pulse generator are counted in groups (e.g., 6 pulses per group) by a "pulse group" counter and a "group of pulses" counter arranged in sequence. When a predetermined number of pulse groups representative of an initial maximum time period have been counted and recorded in a call register, the output of the "pulse group" counter bypasses the "group of pulses" counter to an overtime register.
Also of interest to this invention is the U.S. Pat. No. 3,801,745, issued Apr. 2, 1974, to W. Zuckerman, for timing telephone calls placed through a PBX system. In this patent, pulses from a single pulse generator initially feed to an initial call time period circuit in the form of an adjustable R-C discharge network. When the network times out, it opens a blocking circuit to permit a continuously generated audioalarm signal to be placed on the line. The pulses then feed to an adjustable call overtime R-C discharge network which functions in the same manner to permit periodic audio-alarm signals to be placed on the line until the call is terminated.
Accordingly, the purpose of the subject invention is to provide an economical and accurate electronic telephone call timing system which can be readily wired for producing different initial time and overtime periods in the timing of both local subscriber and local coin telephone calls, and which is capable of producing a plurality of independent time-of-day changes in both the initial time and overtime periods, as desired.